This invention relates generally to microwave circuits, and more particularly the invention relates to signal couplers as used in microwave circuits. Microwave circuits often use signal couplers between circuit sections. Numerous coupling structures have been used in the past, including microstrip coupled lines, stripline coupled lines, and coplanar waveguide coupled lines. The most common form of microwave circuitry is microstrip transmission lines. Microstrip couplers, such as the Lange coupler, are difficult to manufacture in printed circuit boards which have limited and rather course resolution of lines and spaces. This is particularly true when one requires high coupling values on the order of xe2x88x923 dB (i.e., one-half of the incident energy). It has been demonstrated previously that a coplanar waveguide coupler can satisfy the high value coupling while only requiring medium resolution of line widths and gaps. However, a coupler realized entirely in coplanar waveguide form requires microstrip-to-coplanar waveguide transitions which consume space and place restrictions on circuit design.
Frick U.S. Pat. No. 5,629,654 discloses a totally coplanar waveguide construction. However, for many applications it is desirable to have microstrip transmission line input and output ports. Prior art designs require separate microstrip-to-coplanar transitions in order to use a coplanar waveguide coupler with microstrip input and output ports. This adds complexity to circuit design and consumes additional circuit board space.
The present invention is directed to providing a coplanar waveguide coupler for microstrip input/output ports which is space efficient and compatible with printed circuit board manufacturing considerations while providing requisite coupling value, terminal return loss, and isolation.
In accordance with the invention, a coplanar waveguide coupler for microstrip input and output ports in a microwave circuit is fabricated on a printed circuit substrate in which the microstrip input and output lines are on one surface of the substrate, and the coplanar waveguide coupler lines are on an opposite surface of the substrate with vias interconnecting the microstrip ports with the coplanar waveguide coupler lines.
A metal layer is formed on the surface with the coplanar coupler, but is spaced from the coupler. The metal layer functions with the input and output ports in forming the microstrip ground plane.
In a preferred embodiment, first and second metal lines of the coplanar coupler are in spaced parallel alignment, and preferably comprise U-shaped metal traces. A bias metal line can be placed on the surface with the input and output ports which extends across and spaced from the coplanar coupler lines on the opposing surface for providing bias voltage to circuit components mounted on the surface. By grounding the bias metal line, the coplanar coupler can include a partial grounded coplanar waveguide. The amount of the grounded coplanar waveguide center region can be adjusted to optimize the characteristics of the overall coupler.